The present invention relates to a process for the production of a primary aromatic diisocyanate by reacting the corresponding primary aromatic diamine with phosgene in the gaseous phase, in which phosgene and the primary aromatic diamine are reacted within a mean residence time of from 0.05 to 15 seconds and the primary aromatic diamine used contains less than 0.05 mole % of aliphatic amine groups per mole of primary amino groups and no keto groups are present in any aliphatic amine which is present.
Various processes for the production of diisocyanates by reacting diamines with phosgene in the gaseous phase, particularly the phosgenation of aliphatic diamines in the gaseous phase, have already been described in detail in the prior art.
EP-B1-0 289 840 discloses a process for the production of diisocyanates of the general formula OCN—R—NCO, in which R denotes a (cyclo)aliphatic hydrocarbon radical with up to 15 carbon atoms, by phosgenation of the corresponding diamines of the general formula H2N—R—NH2 in the gaseous phase. In this disclosed process, the diamines in the vapor phase, optionally diluted with an inert gas or with the vapors of an inert solvent, and phosgene are heated separately to temperatures from 200° to 600° C. and are continuously reacted in a cylindrical reactor/reaction zone without any moving parts heated to 200° to 600° C. while maintaining a turbulent flow in the reactor/reaction zone. The gaseous mixture leaving the reactor/reaction zone is passed through an inert solvent which is maintained at a temperature above the decomposition temperature of the carbamyl chloride corresponding to the diamine. The diisocyanate dissolved in the inert solvent is subjected to a distillative working-up.
The process described in EP-B1-0 289 840 has been repeatedly modified in further publications both with respect to its range of application and also with respect to apparatus. EP-B1-0 749 958 discloses the reaction of (cyclo)aliphatic triamines with three primary amino groups in the gaseous phase in a tubular reactor at 200° to 600° C. EP-B1-1 078 918 discloses the application of the basic principles of EP-B1-0 289 840 to the gas phase phosgenation of (cyclo)aliphatic diamines with two primary amino groups in the 1,2- or 1,3-position with respect to one another. EP-B1 1 275 639 and EP-B1 275 640 disclose special reactor configurations for an improved intermixing of the reactants fed to the reactor/reaction zone.
The reaction of aromatic diamines with phosgene in the gaseous phase to form the corresponding diisocyanates is also described in the literature.
EP-B1-0 593 334 discloses a process for the production of aromatic diisocyanates in the gaseous phase, in which a tubular reactor is used. In EP-B1-0 593 334 a mixing of the educts without stirring is achieved by a constriction of the walls. The reaction is carried out in the temperature range from 250° to 500° C. The process is however problematic because the mixing of the educt streams solely via a constriction of the tubular wall gives poor results compared to the use of a mixing device. Poor mixing normally leads to formation of an undesirably high amount of solids.
EP-A-0 699 657 discloses a process for the production of aromatic diisocyanates in the gaseous phase in a mixed reactor, which comprises a first homogenizing zone and a second, essentially piston flow downstream zone. Nevertheless, in this process problems also arise due to the formation of solids which block the mixing and reaction equipment and reduce the yield.
There have been a number of attempts to minimize the formation of solids, particularly in the reaction of aromatic diamines with phosgene in the gaseous phase, in order to allow an industrial scale phosgenation of aromatic diamines in the gaseous phase.
EP-B1-0 570 799 discloses a process for the production of aromatic diisocyanates in which the reaction of the appropriate diamine with phosgene is carried out in a tubular reactor above the boiling point of the diamine and within a mean residence time of 0.5 to 5 seconds. According to the teaching of EP-B1-0 570 799, homogenization of the flow in the reaction zone is necessary in order to minimize the undesirable formation of solids and to prevent a back mixing of the components in the reactor/reaction zone. In the process described in EP-B1-0 570 799, the mean deviation from the mean residence time is less than 6%. The maintenance of this residence time is achieved if the reaction is carried out in a tubular flow which is characterised either by a Reynolds number of above 4 000 or a Bodenstein number of above 100.
Measures for averaging out the flow conditions are also the subject-matter of EP-B1-1 362 847. EP-B1-1 362 847 discloses a process for the production of aromatic diisocyanates in the gaseous phase in which the reaction conditions in tubular reactors are improved. In this process, flow-related measures such as the averaging out and centering of the educt streams are used to avoid temperature fluctuations over time and an asymmetry in the temperature distribution. It is temperature fluctuation and asymmetry in the temperature distribution which EP-B1-1 362 847 teaches to be responsible for caking and blockages in the reactor and thus to a reduced service life of the reactors.
EP-A1-1 449 826 teaches that in the reaction of the aromatic diamines with phosgene in the gaseous phase, the reaction of the phosgene with the diamine to form the diisocyanate competes with the subsequent reaction of the diamine with the diisocyanate to form the corresponding urea oligomer. An improved mixing of the phosgene and diamine educts with a simultaneous avoidance of back flow due to eddy effects in the tubular reactor is taught to increase the selectivity of the diisocyanate formation and reduce the formation of urea. In this way, according to the teaching of EP-A1-1 449 826, the amount of condensation product in the tubular reactor can be reduced. The deposition of condensation product on the reactor wall leads to a reduction of the free tubular cross-section, to a gradual increase in pressure in the reactor, and ultimately determines the lifetime of the process. Similarly, EP-A1-1 526 129, DE-A-103 59 627 and WO 2007/028 751 A disclose apparatus-type solutions for improving the mixing of the educts. Flow technology measures to generate angular momentum are taught in EP-A-1 526 129. Concentrically arranged annular nozzles with singular amine feed are disclosed in DE-A-103 59 627, the use a of multiple amine feed is disclosed in WO 2007/028 751 A. Multiple amine nozzles arranged parallel to the axis of a tubular reactor are taught in EP-A1-1 449 826.
WO 2003/045 900 A deals comprehensively with the question of adjusting and controlling the temperature in the production of isocyanates on an industrial scale by a gas phase phosgenation. As is disclosed in WO 2003/045 900 A, in the known processes for the gas phase phosgenation that employ a cylindrical reaction zone, two possibilities for the technical realization are available. The first alternative is to carry out the reaction in a single tubular section, the diameter of which has to be adapted to the production capacity of the plant. According to WO 2003/045 900 A, this concept is disadvantageous for very large production plants because an accurate temperature control of the reaction flows in the core of the flow can no longer be realized by heating/cooling the wall of the tube. Local temperature inhomogeneities can, if the temperature is too high, lead to decomposition of the product, while if the temperature is too low, this temperature can lead to an insufficient conversion of the educts to form the desired isocyanate. The second alternative, to subdivide the reacting mixture into individual partial streams, which are then parallel led through smaller, individual tubes, the temperatures of which can be better controlled due to their smaller diameter is also regarded as a disadvantage by WO 2003/045 900 A. According to WO 2003/045 900 A, the disadvantage of this process variant is that it is relatively susceptible to blockages unless the volume flow through each individual tube is regulated. According to the teaching of WO 2003/045 900 A, the outlined disadvantages can be circumvented advantageously for example with regard to a long or on-stream time of the production plant, by continuous phosgenation of amines in the gaseous phase in a non-cylindrical reaction channel, preferably a plate reactor, which preferably has a height that allows temperature control of the reactants and has a width which is at least twice the height. As WO 2003/045 900 A also discloses, the height of the reaction channel is in general not restricted and the reaction can take place in a reaction channel with a height of, for example, 40 cm. However, there should be a better heat exchange with the reactor walls. Accordingly, WO 2003/045 900 A teaches that the reaction should be carried out in reaction channels of low height.
WO 2007/028 715 A teaches that amines can be converted in a gas phase phosgenation to the corresponding isocyanates if certain requirements are satisfied. Amines which can be converted to the gaseous phase without any significant decomposition are particularly suitable. According to the teaching of WO 2007/028 715 A, suitable amines are those which, during the duration of the reaction, decompose to an extent of 2 mole % or less, more preferably 1 mole % or less and most preferably 0.5 mole % or less under the reaction conditions.
EP-A-1 754 698 teaches that the partial decomposition of the amines employed in the gaseous phase phosgenation to form ammonia has to be borne in mind not only during the reaction, but also has to be taken into account in vaporization of the amines. Ammonia formed by partial decomposition of the amines during vaporization not only reduces the yield, but in the subsequent phosgenation it also forms ammonium chloride deposits downstream in connected piping and apparatus. The plants then have to be cleaned relatively often, resulting in corresponding production losses.
Despite the attempts to optimize the reaction of aromatic amines with phosgene in the gaseous phase and thereby minimize the formation of solids that can be observed in the reaction of aromatic diamines with phosgene in the gaseous phase, there remains a need to improve the gas phase phosgenation of aromatic diamines to permit an industrial scale phosgenation of aromatic diamines in the gaseous phase.